Approval process : locking functionality to prevent 2 user from proceesing approval at same time

Hi All,

I have developed approval workflow for inventory management using standard BOR.and it has 3 levels of approval.

and multiple approvals at one step.

I want to prevent more than one users from processing the same workitem at the same time and give message that this work item is locked for processing by user ---.

Pls help.

Thanks,

Arpita